The effect of branched structure on the lamellae evolution under uniaxial extension and SCG resistance of polyolefins

A set of unimodal and bimodal high density polyethylene (HDPE) pipe resins of different grade were characterized to investigate the relation between branched structure and resistance to SCG. The results showed that the SCB of bimodal PE100RC was more likely to incorporate into long chains and the entanglement of tie molecules was higher, thus making thicker lamellae and more complete lamellae network. The strain hardening (SH) modulus proved bimodal PE100RC had better long term performance compare with unimodal and bimodal PE100. Besides, small-angle/wide-angle X-ray scattering (SAXS/WAXS) presented the lamellae evolution of bimodal PE100RC and PE100 under uniaxial extension. In the transition of shish-kebab to fibrillar crystals, the orientation increased with the rising of strain and presented a two-stage process with the turning point at the strain of about 1.0. And the long period curves exhibited a three-stage process which were yeilding, softening and hardening stage. In the yeilding region, the long period of PE100RC grew at a faster rate compared with PE100 as the amorphous region of PE100RC was easier to stretch. At the end of yield region, the lamellae thickness of PE100 was smaller than before, while that of PE100RC became larger. It can be inferred that in PE100, only fragmentation of lamellae can be observed, while in PE100RC, the recrystallization as well as destruction of lamellae occurred.